U.S. Pat. Nos. 3,288,846 and 3,455,675 describe a process of phosphonomethylation of IDA performed by preparing IDA hydrochloride to which there are added further hydrochloric acid and phosphoric acid, followed by formaldehyde.
Italian Patent No. IT-1 118 553 describes an alternative process in which phosphorous trichloride is used as the phosphorous source. The phosphorous trichloride is hydrolysed in water to produce a solution wherein the reaction products, phosphoric acid and hydrochloric acid, are present in a stoichiometric molar ratio of the hydrolysis reaction of 1:3. The solution obtained in this way is then reacted with iminodiacetic acid and then formaldehyde.
Although phosphorous trichloride is an economical source of phosphorous, it has been established that the hydrolysis in water of phosphorous trichloride, which is an extremely exothermic reaction, is difficult to control and easily leads to byproducts, particularly to oxidation products of the phosphoric acid.
Furthermore, the large amount of hydrochloric acid present in the phosphonomethylation product renders the recovery of PMIDA complicated and expensive, requires a high consumption of energy, does not enable commercial hydrochloric acid to be recovered by distillation since the latter is highly contaminated by formaldehyde, and produces a considerable quantity of liquid refluents which are very expensive to process.
The object of the present invention is to provide an improved process which is easier and more economical, as compared to known processes.